Which acellular infectious agent is described as a tiny, nonliving particle that invades and then reproduces inside a living cell?

• A. Prion
• B. Virus
• C. Viroid
• D. Bacterium

Answer: (B)

Verified step by step guidance

1. Understand the key characteristics described: the agent is acellular, tiny, nonliving, and requires a living cell to reproduce.
2. Recall definitions of each option: A prion is an infectious protein without nucleic acids; a viroid is a small infectious RNA molecule affecting plants; a bacterium is a living cellular organism.
3. Recognize that only viruses fit all criteria: they are acellular particles composed of nucleic acid and protein, nonliving outside a host, and reproduce by invading living cells.
4. Eliminate options that do not match all criteria: prions do not contain nucleic acids and do not reproduce inside cells in the traditional sense; viroids infect plants specifically; bacteria are living cells themselves.
5. Conclude that the correct acellular infectious agent described is a virus.